Recent studies led by the PI demonstrated that activation of PTH receptor signaling in osteocytes is sufficient to increase bone mass and the rate of remodeling, two recognized actions of PTH on the skeleton; positioning osteocytes as crucial target cells of the hormone. The long term goal of this research is to determine the contribution of osteocytes to the regulation of skeletal homeostasis. The specific goal of this proposal is to reveal the mechanisms by which osteocytes induce bone resorption in response to PTH. Mice treated with PTH or expressing a constitutively active PTH receptor in osteocytes exhibit increased osteoclasts and elevated bone resorption. Conversely, mice lacking the PTH receptor in osteocytes exhibit decreased bone resorption. PTH-induced resorption is due to upregulation of the RANKL gene and requires extracellular matrix degradation by matrix metalloproteinases (MMPs). Although it is recognized that the pro-osteoclastogenic action of PTH is mediated by cells of the osteoblastic lineage, the precise stage of differentiation of the PTH target cell remains unknown. Recent evidence indicates that osteocytes are an important source of RANKL; and that they secrete MMP13, which might facilitate osteoclast recruitment and resorption, and express membrane-bound MMP14 (or MT1-MMP), which remodels the pericellular matrix thus maintaining the osteocyte canalicular network and also induces RANKL shedding. Based on this evidence and findings showing that the expression of RANKL, MMP13 and MMP14 is elevated by activation of PTH receptor signaling in osteocytes, we hypothesize that direct effects of PTH on osteocytes increase RANKL and MMPs, which in turn act as osteocyte-derived factors to increase osteoclast formation and activity, and bone resorption. This hypothesis will be tested by pursuing two specific aims that combine in vivo and in vitro approaches and use novel genetically modified mice and murine and human osteocytic cells and murine primary osteocytes. Aim 1 will examine the contribution of membrane-bound and soluble RANKL to resorption induced by PTHR1 activation in osteocytes. Aim 2 will determine the contribution of osteocyte-derived MMPs to PTH-induced resorption. Successful completion of the proposed studies will positively impact basic bone biology and pathophysiology by advancing our understanding of the mechanisms by which PTH receptor signaling in osteocytes regulates the function of the cells that remodel bone, osteoclasts and osteoblasts, in health and disease.